Past droughts and floods

Droughts

The definition of ‘drought’ is complex, and dependant on
different factors such as location and climatic zone. Droughts are generally
accepted to be prolonged periods of abnormally dry weather, with scarcity
of moisture having an adverse effect on vegetation and animal life in
an area. Droughts have obvious direct effects on freshwater ecosystems
by reducing water levels and availability. However, droughts also induce
damaging indirect effects, such as the conversion of a flowing stream
or river into a series of stagnant pools which may lower water quality
with lower oxygen levels and higher levels of dissolved organic matter.

Drought often carries with it negative connotations, with images of
dying sheep and cattle. However, we must recognise that drought events
are a natural part of many ecosystems and may play an important role
in the functioning of these ecosystems. A Symposium on the Role of Drought
in Aquatic Ecosystems was held in Albury, Australia in February 2001. View
the synopsis of the Symposium here. {http://enterprise.canberra.edu.au/WWW/www-directreps.nsf/0/002effbc275fcc05ca256cd2001f32fc/$FILE/Drought+symposium+synopsis+-+Feb+2001.pdf}

Prior to the 1860s droughts were scarcely noticed as most settlers were
living in areas with reliable rainfall. It was the drought of 1864-65
which led to the drawing of Goyder’s ‘line of rainfall’ indicating
the limit of the rainfall which separated lands suitable for agriculture
from those fit for pastoral use only. It marked areas of reliable and
unreliable annual rainfall, and was pivotal in raising awareness for
South Australian settlers of the limits for successful agriculture.

The current drought which is affecting the Murray-Darling system is
said to be equal to the worst droughts of the last century, and is shaping
up to be worse than the last experienced in 1983.

Over the last 100 years since accurate records have been kept, South
Australia has experienced several severe droughts which affected the
River Murray in South Australia in the following years:

1884-86

1895-98

1901-03

1911-15

1927-29

1943-46

1959

1961

1967

1976-77

1982-83

The early push for irrigation followed some very serious droughts, with
many contemporaries of the time concluding that the only way to secure
water for development was to dam the rivers.

In its natural state, the River Murray was quite different from the
present day regulated river. During severe droughts it was sometimes
reduced to a chain of waterholes which made the river too unreliable
to enable intense settlement in South Australia.

As part of any natural river feature, droughts and floods are part of
an ancient, natural cycle. However, where the Murray would naturally
experience drought flows once every 20 years, it now experiences artificially
induced drought conditions in six of every 10 years.

Unlike the early pastoralists, hydrologists today are aware that the
frequent droughts which occurred in the 19th century, and the above-average
rainfall in the second part of the twentieth century, are a feature of
the highly variable climate in Australia.

Floods

Generally speaking, flooding in the River Murray is caused by climatic
events beyond South Australia, such as abnormal snow melt in the Snowy
Mountains in New South Wales and Victoria.

Apart from periods of major flood, the River Murray now runs low in
winter, spring and early summer, when it previously flooded regularly,
and runs high in late summer and the early autumn, when previously it
often ran low. This impact is reduced further downstream due to the flow
coming from rivers such as the Loddon, which do not have major storages
to capture winter rain (the Loddon system comprises storages, weirs and
connecting channels which integrate the supply of water from the Campaspe
and Loddon River catchments. These have been built gradually over
decades beginning in the late 1880s, the early 1940s and through to the
mid 1950s and 1960s. To find out more about some water storage
systems up the Murray go to:

Since accurate records have been kept, floods have occurred in the Murray-Darling
system in:

1867 Disastrous flood on the Murrumbidgee River.
For the Murray, a 1 in 90 year flood, with the 4th highest level ever
recorded at Albury (NSW) and 2nd highest at Echuca (Vic)

1870 First officially recorded flood. Peaked 11M
at Morgan. It washed away homesteads at Craignook [117ml] and Murbko
[177ml]

1890 Darling flood peaked 8.25M at Morgan.

1917 Flood reached 9.7M at Morgan.

1931 Flood peaked 9.8M at Morgan and breached some
levees.

1952 Flood peaked 8.6M at Morgan.

1955 Flood peaked 8.3M at Morgan and pre-empted
the 1956 flood.

1956 Flood peaked 12.3M at Morgan, breached all
levees and was the highest flood recorded since white settlement. Traditionalists
argue that it was only the 'locks' that made it higher than the 1870
flood.

1974 Flood peaked 8.5M at Morgan (old annual floods
peaked about 5-7M, but by 1974 the Snowy Mountains Water Management
Scheme, designed to 'even out' the Murray's fluctuations, was fully
operational.)

In South Australia, a River
Murray flood is considered exceptional when the water level is
more than 5 metres above pool level. In South Australian recorded history
this has occurred in 1931, 1956, 1973 and 1974. {http://www.atlas.sa.gov.au/atlas1986/2ENVIRONMENT_RESOURCES/10NATURAL_HAZARDS.cfm}

The 1956 River Murray flood is considered to be the greatest natural
catastrophe in South Australia’s history and is the largest flood
ever recorded in the state. It occurred as a result of excessive and
late rains in the western districts of Queensland, and heavy rains commencing
three months earlier than usual in the Murray catchment areas. The Darling
and the Murray were both in high flood at the same time. During floods
the waters in the River Murray system spread out below Tocumwal in NSW
and engulf the Edward and Wakool rivers, forming one huge lake which
helps to control flooding further downsteam by acting as a natural reservoir.
In 1956 the usual spread of waters below Tocumwal could not hold the
quantity of water and the swollen river raced on its way though South
Australia (Mortimer, p 7, 1985) causing widespread damage to agricultural
properties and townships. This flood resulted in the construction of
the Menindee Lakes storage areas to store high Darling River flows.